1. Field of the Invention
This invention relates to an apparatus for pulling up crystal bodies, such as silicon single crystals, by the CZ method, and especially relates to an apparatus for safely pulling up heavy crystal bodies.
2. Description of Related Art
In the process of pulling up crystal bodies, such as silicon single crystals, by the CZ method, a seed crystal installed within a seed-crystal holder is suspended by wires and dipped into the melt. Then, the seed crystal is pulled up from the melt by rotating and winding up the wires.
In recent years, the need for large-diameter semiconductor wafers is increasing. Following the enlargement of crystal bodies, weight of an individual single crystal is increasing. For example, the weight of a crystal body with 300 mm diameter can reach a value of 150 Kg. In the process of pulling up conventional lightweight crystal bodies, the weight of the crystal body being pulled up is supported by the connection between the seed crystal and the crystal body only. However, it is difficult to support the upper part of the heavy crystal body by only the connection between the seed crystal and the crystal body. Especially the load is concentrated at a so-called dash neck, a small-diameter portion formed so as to prevent any dislocation caused by the seed crystal from being transmitted to f the crystal body on being pulling up, thereby the load may cause the dash neck to break and results the crystal body being dropped. This not only incurs the loss of the crystal body but also considerable damage to the pulling up apparatus.
To prevent the dropping of a crystal body, a crystal pulling up apparatus was disclosed in Japanese Patent Publication Laid Open (JP-A-) HEI 7-10300. In the single-crystal pulling up apparatus disclosed in Japanese Patent Publication(JP-B) HEI 7-10300, a necked portion is formed on the top of a crystal body beforehand, and the necked portion is clamped by several "V shaped" holding levers extending from the seed crystal holder.
However, for a crystal body of 300 mm diameter, the distance from the seed crystal holder to the necked portion is over 500 mm. Therefore, the above "V shaped" holding levers need to be longer than 500 mm. At this length, it is difficult to achieve enough strength to support the weight of the crystal body.
Furthermore, it is quite difficult to maintain an unvarying distance from the seed crystal holder to the necked portion to match the length of the holding levers.